Ghost-peak suppression in ultrafast two-dimensional NMR spectroscopy.

Two-dimensional (2D) spectroscopy is central to many contemporary applications of NMR. Recently, we have introduced a new approach whereby 2D NMR spectra can be collected within a single scan. This methodology employs a magnetic field gradient in order to spatially encode the time evolution occurring along the indirect dimension. The discrete nature of the t1 incrementation and its one-to-one correspondence with the spatial encoding, may lead to a number of artifacts. Most notable among these is a periodicity of the spectral peaks that are observed along the indirect axes. The appearance of such 'ghost-peaks', which may sometime coincide with genuine cross-peaks, could hamper a proper interpretation of the spectra. This contribution reviews the origin of such multiple resonances, and proposes a procedure for their elimination based on the acquisition of a small number of complementary scans. Such complementary scans can be simultaneously employed for the sake of phase-cycling out other unwanted signals, and improve the overall indirect-domain spectral resolution. Brief mathematical descriptions of the ghost-peak generation and ghost-peak suppression mechanisms are described, followed by experimental tests on a number of samples using various pulse sequences.